This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. West Nile (WNV) is an emerging virus that was introduced in the United States on 1999 and has expanded rapidly throughout the country. This virus caused more than 12,000 reported human cases in the last two years with clinical symptoms that ranged from a mild febrile illness to a potentially fatal encephalitis. Little is known about WNV transmission in dengue-endemic regions, such as Central and South America and the Caribbean islands. Epizootic studies demonstrated that the virus is circulating in horses, birds and probably humans but symptomatic WN cases have not yet been reported in these countries. Since there is extensive cross reactivity between DEN and WNV antigens, our hypothesis is that individuals that are immune to dengue viruses (DENV) may be cross-protected against WNV illnesses. To prove this hypothesis, we will perform animal studies using Balb/c mice. We will first evaluate the susceptibility and kinetics of WNV infection in this mice strain (specific aim 1). Then, two separate experiments will be performed to test if cross-protection against WNV can be achieved by active immunization with DENV or a DEN-2 DNA vaccine candidate (specific aim 2) or passive immunization with human serum collected from primary or secondary DENV infections (specific aim 3). Immune responses against DENV and WNV will be measured by indirect immunofluorescence assays, neutralization tests and cytokine assays using the Bio-plex system. Mice morbidity and mortality after WNV challenge will be monitored for 28 days and viral loads in blood and brain will be determined by Taqman assays. If cross-protection is achieved, these experiments will allow us to determine the type and duration of the immune responses that may correlate with protection. Future studies will be aimed to determine the DEN viral proteins that may play an important role in cross-protection. Results from this and future studies in monkeys will provide valuable information to improve the design of broad spectrum vaccines that could potentially protect against both pathogens, DENV and WNV.